I've created a couple of models with a method that makes sense to me, but it's not working.  I would like to have a master model of an assembly that I can then copy and change a few overall dimensions creating a new custom-sized model.  It's not discreet standardized sizes that I'm trying to set up, but rather an infinite number of custom sizes.  I created a part called "internal dimensions" that is just a basic box with length width and height dimensions representing the interior size of my assembly.  I have set up part dimensions from within the master assembly that relate to the dimensions of my "internal dimensions" part via equations.  This has worked sort-of, but at some point on a couple of my models the dimensions stopped updating and I can't figure out why.  The dimensions then say they're "read only" and I can't go in and look at the equation to figure out what's wrong.  Is what I'm trying to do something that others do successfully or am I just trying to do something that shouldn't be attempted?  Seems to me it ought to work and it would be a huge time-saver if it were working the way I've envisioned.  Any helpful hints would be appreciated!